An extreme following > followers ratio. An account that is following hundreds of people, but has only a handful of followers, is in many cases a spam account.
BUT by no means is this a straight indicator, merely a good warning sign to keep an eye out.
Obviously, 'people' you don't know personally sending you very short direct messages with a shortened url link are suspect (have had plenty of those, which you can easily block from then on).
Edit: just stumbled on this service today via Lifehacker: Fruji. The basic features are free to use, among which an analysis that determines Twitter accounts with a high 'marketing' factor, or put differently: whose primary aim is to build a marketing network.